The present invention relates to a novel seedless watermelon having an average fruit weight of less than 12.0 pounds. The present invention also relates to a seedless watermelon seed, a seedless watermelon plant, seedless watermelon variety, and a seedless watermelon hybrid and to methods of producing seedless watermelon fruit.
Watermelon belongs to the family Cucurbitaceae. Watermelon is commercially grown from either seed or transplants. Citrullus is a member of the family Cucurbitaceae. The Cucurbitaceae is a family of about 90 genera and 700 to 760 species, mostly of the tropics. The family includes pumpkins, squashes, gourds, melons, cucumber, watermelon, loofah, and several weeds. A bitter-fruited form of Citrullus vulgaris appears to be the ancestor of the cultivated watermelon.
Successful watermelon production depends on attention to various cultural practices. This involves soil management practices with special attention to proper fertilization, crop establishment with appropriate spacing, weed control, the introduction of bees for pollination, and suitable pollenizers for seedless watermelon, irrigation and pest management. Watermelon fruit size and shape; rind color; thickness and toughness; seed size, color and number; and flesh color, texture, soluble solids and freedom from fruit defects are all important characteristics to be considered in selection of watermelon varieties. In addition, seedless watermelons should be free of hard seeds and have undeveloped seeds that are small and innocuous.
Watermelon crops can be established in the field from seed or from transplants. Transplanting is becoming more common because transplanting usually results in earlier crops than those that are direct seeded. Transplants are used extensively to establish seedless watermelon plantings. Diploid and triploid watermelon crops can be established easily with high quality transplants. Transplanting helps achieve rapid, complete plant stands, especially where seed costs make direct-seeding risky and expensive, as is the case with seedless watermelons. Most watermelon growers purchase plants from plant growing experts who may arrange for transport to the field location.
For triploid seedless watermelon production, fruit set and enlargement is dependent upon growth regulators from the pollen grains and from embryos in developing seeds within the fruit. Inadequate pollination results in triploid watermelon fruit that are triangular in shape and of poor quality. Inadequate pollination may increase the incidence of hollowheart. Triploid watermelon flowers do not produce sufficient viable pollen to induce fruit set and development. Therefore, pollen from a normal diploid seeded watermelon variety must be provided. Planting the diploid pollenizer variety in the outside row of the field and then every third row is recommended. As an alternative, the pollenizer variety has been planted every third plant in each row but this makes harvesting of the triploid fruit difficult because mixed diploid and triploid fruit must be separated. This also makes planting difficult because blanks must be left where the diploid should go. Maintaining the rotation of three triploid to one diploid is not easily accomplished.
Currently, it is important to use a diploid pollenizer variety that is marketable because between one-quarter to one-half of all watermelons produced in the field will be of this variety. The rind pattern and/or shape of the seeded pollenizer fruit should be distinguished easily from that of the triploid fruit to reduce confusion at harvest.
It is important that pollen from the diploid pollenizer variety is available when female blossoms on the triploid plants are open and ready for pollination. If planted too early, the diploid variety can set fruit and stop producing male blossoms while the triploid variety is still producing many female blossoms. If planted too late, the triploid will be ready to set fruit but not enough pollen will be ready to provide fruit set.
Watermelon plants develop several vigorous and far-reaching vines, thus requiring large amounts of space for optimum growth and fruit development. Watermelons have been seeded with about two to about four feet between plants in rows about six to about 15 feet apart. This wide spacing requires larger field sizes. Also, the wide spacing provided less interplant competition for water. Cultural practices such as irrigation and polyethylene mulch have led to the use of higher plant populations. Row spacing of 6–8 feet apart and plant spacing of 2–4 feet are common. Often, with close plant spacing, the individual plant sets fewer fruits, which still reach normal size.
Watermelon plants usually have separate male and female flowers but sometimes produce perfect flowers. To achieve fruit set, pollen from the male flower must be transferred to a female flower on that plant or another plant in the field. This pollen transfer is accomplished by several naturally occurring insects, but most effectively by the honeybee. Poor or ineffective pollination of watermelons results in bottle-neck fruits of long-fruited watermelon varieties. In round-fruited varieties, poorly pollinated fruits can be flat-sided or misshapen.
Watermelon has small flowers. Flowering begins about 8 weeks after seeding. Flowers of watermelon are staminate (male), perfect (hermaphroditic), or pistillate (female), usually borne in that order on the plant as it grows. Monecious types are most common, but there are andromonoecious (staminate and perfect) types, mainly the older varieties or accessions collected from the wild. In many varieties, the pistillate or perfect flowers are borne at every seventh node, with staminate flowers at the intervening nodes. The flower ratio of typical watermelon varieties is 7 staminate to 1 pistillate, but the ratio ranges from 4:1 to 15:1.
Watermelon is the only economically important cucurbit with pinnatifid (lobed) leaves; all of the other species have whole (nonlobed) leaves. The leaves are pinnately divided into three or four pairs of lobes, except for an entire-leaf (nonlobed) gene mutant controlled by the nl (nonlobed) gene. Watermelon growth habit is a trailing vine. The stems are thin, hairy, angular, grooved, and have branched tendrils at each node. The stems are highly branched and up to 30 feet long, although there are dwarf types (dw-1 and dw-2 genes) with shorter, less-branched stems. Roots are extensive but shallow, with a taproot and many lateral roots.
Vine length of watermelon varies from dwarf to long. For example, ‘Charleston Gray’ and ‘Jubilee’, large-fruited varieties, have vines up to 30 feet long. Short or medium length vines are well suited to varieties with small or medium sized fruit. For example, ‘Sugar Baby’, ‘New Hampshire Midget’, and ‘Petite Sweet’ are short vined, having vine lengths of between about six to about 12 feet and ‘Crimson Sweet’ has intermediate vine length.
Dwarf mutants have been discovered in watermelon. Two genes cause dwarfing when they are in homozygous recessive condition: dw-1 and dw-2. ‘Kengarden’ has the genotype dw-1 dw-2. Another gene mutant (Japanese Dwarf, dw-2 dw-2) has increased branching from the crown.
Fruit size is an important consideration in a breeding program since there are different market requirements for particular groups of shippers and consumers. The general categories are: icebox (<12 lb), small, sometimes called pee-wee (12–18 lb), medium (18–24 lb), large (24–32 lb), and giant (>32 lb). Fruit size is inherited in polygenic fashion, with an estimated 25 genes involved. Shippers in the United States work with particular weight categories, such as 18–24 lb for seeded and 14–18 lb for seedless. Researchers have developed three diploid seeded varieties with small fruit: New Hampshire Midget, Minilee and Mickylee. (Barnes, et al., Australian Journal of Experimental Agriculture. 1994. 34(5):673–679.)
The commercially available seedless watermelons have a round to round/oval to blocky shape with an average weight between 15–20 pounds. The length/width ratio (L/W ratio) for these commercial hybrids ranges from 1.2 (10×8.5 inches) to 1.7 (11.5×7 inches). There may be exceptional circumstances where the weight can go up to 32 pounds in the larger extreme; and there may be also cases where fruits weigh as little as 12–15 pounds in the smaller extreme. However the standard weight in the seedless watermelon trade in the U.S. is that of 15–20 pounds. These ranges of measurements and ratios apply for hybrid seedless watermelons grown in commercial fields and under conventional spacing between the rows and within the rows.
Seedless triploid varieties are produced by crossing a tetraploid (2n=4x=44 chromosomes) inbred line as the female parent with a diploid (2n=2x=22) inbred line as the male parent of the hybrid. The reciprocal cross (diploid female parent) does not produce seeds. The resulting hybrid is a triploid (2n=3x=33). Triploid plants have three sets of chromosomes, and three sets cannot be divided evenly during meiosis. This results in nonfunctional female and male gametes although the flowers appear normal. Since the triploid hybrid is female sterile, the fruit induced by pollination tend to be seedless. Unfortunately, the triploid has no viable pollen, so it is necessary to plant a diploid variety in the production field to provide the pollen that stimulates fruit to form. Usually, one third of the plants in the field are diploid and two thirds are triploid, although production has been observed with as little as 20% diploids. Varieties should be chosen that could be distinguished easily so the seeded diploid fruit can be separated from the seedless triploid fruit for harvesting and marketing.
Most of the tetraploid lines being used by the seed industry have gray rind so that, when crossed with a diploid line with striped rind, it will be easy to separate self-pollinated progeny (which will be seeded fruit from the female parent line) from cross-pollinated progeny (which will be seedless fruit from the triploid hybrid). The grower may discard the gray fruit so they are not marketed as seedless watermelons by mistake. For example, if there is 4% of the fruit from the inbred parent then 4% of total fruits will be unmarketable and reduces marketable yield.
A smaller fruited seedless watermelon, if available, would be desirable for certain segments of the consumer market.